Ferroelectric based capacitors offer potential advantages in integrated circuit design. Many integrated circuits incorporate capacitors. For example, DRAM memory cells store a binary value by storing charge on a small capacitor. These capacitors consist of two conductors separated by a dielectric material such as silicon dioxide. The capacitance of the capacitor determines the length of time between refresh cycles. As circuit designers improve these circuits by reducing the size of the components on the silicon substrate, the size of the capacitors is also reduced. As a result, the capacitance of the capacitor is also reduced. The decrease in capacitance requires the DRAM cells to be refreshed more often. There is some minimum retention period below which refresh cycles are inoperative; hence, it would be advantageous to provide a capacitor which is small in size while providing greater capacitance than those currently constructed utilizing silicon compounds as the dielectric.
Ferroelectric materials having very high dielectric constants are known to the art. Lead lanthium zirconate titanates (PLZT) having dielectric constants in excess of 500 have been demonstrated. Hence, such materials could provide significant improvements in integrated circuit capacitors. Unfortunately, attempts to utilize ferroelectric capacitors in integrated circuits have suffered from a number of problems.
A ferroelectric capacitor typically comprises two conductors which sandwich a layer of ferroelectric material. The capacitor is constructed in three steps. First, the bottom conductor is deposited on the integrated circuit substrate. Next, the ferroelectric material is deposited as a thin film on top of the first conductor. The deposition requires processing temperatures in excess of 600.degree. C. Third, the top electrode is deposited on the ferroelectric layer. Fifth, connections must be made to the top and bottom electrodes. The connections to the bottom electrode must be accomplished with the aid of a via hole.
If the top and bottom electrodes are constructed from metallic layers such as metallic gold or platinum and the ferroelectric material is PLZT, the resultant capacitor exhibits an aging effect. The Pt/PLZT interface forms a diode-like junction due to a large band gap work function difference between the interface of the metal and ceramic. This interface has a tendency to trap charges which can not make it over the diode barrier or across the ceramic layer to balance out the charge in the ceramic at the metal interface. This trapped charge induces fatigue which limits the lifetime of the capacitor.
It has been suggested that these aging effects can be reduced by the use of ohmic contacts such as the metallic oxides. For example, Indium Tin Oxide or Ruthenium Oxide for the top and bottom electrodes have been suggested. The ohmic contacts reduce the Schottky barrier in the ferroelectric ceramic. However, to construct a capacitor with metallic oxides for both electrodes requires that the PLZT ceramic be deposited on the metallic oxide in question. Unfortunately, it is difficult to obtain satisfactory deposition of the PLZT material on a bottom electrode constructed from these oxides. Without proper crystallization of the PLZT, the resulting capacitor will leak sufficient charge to render it unsatisfactory for most uses.
Yet another problem with the construction of ferroelectric capacitors is the adhesion of the bottom electrode to the integrated substrate during the ferroelectric deposition steps. As noted above, the ferroelectric deposition steps require temperatures in excess of 600.degree. C. The thermal expansion and contraction during these processing steps often leads to separation of the bottom electrode from the semiconductor substrate when metallic platinum electrodes are used.
Broadly, it is the object of the present invention to provide an improved electrode/PLZT structure which provides the benefits of an ohmic contract while allowing satisfactory deposition of the PLZT material on the electrode during the fabrication of the device.
It is a further object of the present invention to provide a bottom electrode that is more resistant to separation from the underlying substrate during PLZT deposition.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the present invention and the accompanying drawings.